The present invention relates to the detection of trace gas species such as explosives, drugs and steroids with high selectivity and specificity and, more particularly to tunable laser-based systems for trace detection of nitrogen dioxide.
The real-time detection of trace gases in the parts-per-billion (10−9) and parts-per-trillion (10−12) levels is of great interest in a wide range of fields, including environmental science (e.g., study of complex chemical reactions that take place in the atmosphere, particularly in the presence of solar radiation) and air quality control (e.g., for compliance with Environmental Protection Agency regulations), defense and homeland security (e.g., for the detection of trace amounts of explosive compounds), non-invasive medical diagnostics (e.g., breath analysis), detecting trace impurities in semiconductor material processing and device fabrication, in the food industry (e.g., monitoring ethylene), and optimizing combustion processes and minimizing pollution emissions, to name a few. See the following articles that describe these prior concerns: “Primary National Ambient Air Quality Standards for Nitrogen Dioxide; Final Rule,” Federal Register, Vol. 75, No. 26, 6474-6537, Feb. 9, 2010; United States Environmental Protection Agency, “National Air Quality Status and Trends Through 2007, EPA-454/R-08-006”, United States Environmental Protection Agency, Washington D.C., (2008); J. Hildenbrand, et al., Explosive detection using infrared laser spectroscopy,” Proc. SPIE 7222, 72220B-1-72220B-12 (2009); T. H. Risby, S. F. Solga, “Current status of clinical breath analysis,” Appl. Phys. B 85, 421-426 (2006); G. M. Mitchell et al., “Trace Impurity Detection in Ammonia for the Compound Semiconductor Market,” Semicon West, San Francisco, Calif., Jul. 17-21, 2002; and A. Arnold et al., “Laser in situ monitoring of combustion processes,” App. Opt., 29, 4860-4872 (1990).
Laser-based techniques are well suited for trace gas species detection because of their ability to provide real-time monitoring capabilities with a high degree of sensitivity and selectivity. In particular, quantum cascade lasers (which emit in the mid-infrared region covering 4-24 μm) are especially attractive for this task because they provide access to the fundamental rotational-vibrational transitions of molecular species. See, A. A. Kosterev et al., “Chemical Sensing with pulsed QC-DFB lasers operating at 6.6 μm” Appl. Phys. B. 75, 351-357 (2002); F. K. Tittel et al., “Recent Advances in Trace Gas Detection Using Quantum and Interband Cascade Lasers,” Rev. of Laser Eng. 34, 275-282 (2006); A. Kosterev et al., “Application of quantum cascade lasers to trace gas analysis,” Appl. Phys. B 90, 165-176 (2008); R. F. Curl et al., “Quantum cascade lasers in chemical physics” Chem. Phys. Lett. 487, 1-18 (2010); and a recent comprehensive review article by the present inventor, Gottipaty N Rao along with A. Karpf, “External cavity tunable quantum cascade lasers and their applications to trace gas monitoring,” Applied Optics, Vol. 50 Issue 4, pp. A100-A115. (2011).
As noted in these articles, quantum cascade lasers have been used to detect several trace gasses, including CO, CO2, NO, NO2, NH3, CH4 and N2O, as well as explosive compounds such as TNT. See the Hildenbrand et al. article. A reliable NO2 monitor capable of high sensitivity and selectivity would be valuable for monitoring atmospheric air quality (to meet EPA air quality standards, and monitor the formation of photochemical smog, tropospheric ozone, and automobile and industrial emissions), as well as for the real-time study of the complex photochemical reactions that NOx gases undergo in the atmosphere.
A variety of spectroscopic techniques have been developed for detection, each having its own merits and limitations. The spectroscopic techniques that are commonly employed include, absorption spectroscopy using long pass absorption cells such as multipass and Herriott cells, optical cavity methods (cavity ring-down spectroscopy, off-axis integrated cavity output spectroscopy), photo-acoustic and quartz-enhanced photo-acoustic spectroscopy, and Faraday rotation spectroscopy. Various data processing and analysis procedures are followed such as frequency modulated spectroscopy techniques to improve the signal to noise ratio and multiple line integrated absorption spectroscopy to improve the sensitivity of detection. The current status of much of this work was presented in the recent review article by the present inventor Gottipaty N Rao and by A. Karpf, and the conference presentations Gottipaty N Rao et al., “Sensors at ppb Sensitivity or Better Based on Multiple Line Integration Spectroscopy Techniques,” Laser Applications to Chemical, Security and Environmental Analysis (LACSEA) 2010 paper: LPDP2; Gottipaty N Rao et al., “A Trace Gas Sensor at ppb Sensitivity Based on Multiple Line Integration Spectroscopy,” Conference on Lasers and Electro-Optics (CLEO) 2010 paper: JWA60, as well as the inventor's co-pending U.S. patent application Ser. No. 12/878,553 filed on Sep. 9, 2010, and which is incorporated herein in its entirety.
External cavity tunable quantum cascade lasers are quite compact, operate at room temperatures, have large operating lifetimes, require low power levels for operation, provide reasonably high output powers with a narrow laser line width, and can be operated over a widely tunable range (hundreds of wave numbers), which makes them well suited for trace gas monitoring applications in real time. They are also amenable to fiber optic technology and therefore can be employed for remote monitoring applications. Trace gas detection using laser absorption spectroscopy is based on recording the change in intensity of laser radiation as it passes through a region containing the sample of interest. In this technique, often the laser is rapidly swept across specific molecular transitions of interest, the spectra are synchronously added and averaged, and compared with the molecular cross-section data or employ secondary calibration procedures to obtain the concentrations. In order to detect very low concentration species in the ppb level or lower, multi-pass cells can be employed to increase the path length and improve the sensitivity of detection. Using multipass optical cells, one can reach path lengths in the hundreds of meters range; however, the volume of these cells is large (typically about 1 l). The main difficulty with multipath cells is that they are bulky, involve careful cavity adjustments and are sensitive to vibrations which are potential limitations for field applications. Fabry-Perot optical cavities provide long path lengths on the order of several km in a small effective volume. See A. O'Keefe et al., “CW Integrated Cavity Output Spectroscopy,” Chem. Phys. Lett., 307, 343-349 (1999); R. Engeln et al., “Cavity enhanced absorption and cavity enhanced magnetic rotation spectroscopy, Rev. Sci. Instrum. 69, 3763 (1998); J. B. Paul et al., “Ultrasensitive absorption spectroscopy with a high-finesse optical cavity and off-axis alignment,” Appl. Opt. 40, 4904-4910 (2001); and G. Berden et al., “Cavity ring-down spectroscopy: Experimental schemes and applications,” Int. Reviews in Physical Chemistry, Vol. 19, No. 4. 565-607 (2000). In this technique, the laser is coupled to a high finesse optical cavity (formed by highly reflective, low-loss dielectric mirrors) so that a large amount of light energy builds up within the cavity. In cavity ringdown spectroscopy (CRDS), one interrupts the laser beam and measures the exponential decay of the light exiting the cavity (cavity ringdown time) with and without the gas sample. While, CRDS offers high sensitivity of detection and provides an absolute value of the concentration of the species (i.e., no need for secondary calibration procedures), it is susceptible to vibrations and requires stringent cavity resonance conditions.